ASTPlus.java

public class ASTPlus implements ASTNode {

        ASTNode lhs, rhs;

        public ASTPlus(ASTNode l, ASTNode r) {
                lhs = l;
                rhs = r;
        }

        @Override
        public IValue eval(Environment<IValue> e) throws InterpreterError {
                IValue v1 = lhs.eval(e);
                IValue v2 = rhs.eval(e);

                if (v1 == null || v2 == null) {
                        throw new InterpreterError("null value in + operator");
                }

                if (v1 instanceof VInt && v2 instanceof VInt) {
                        int i1 = ((VInt) v1).getval();
                        int i2 = ((VInt) v2).getval();
                        return new VInt(i1 + i2); 
                }

                if (v1 instanceof VString || v2 instanceof VString) {
                        String s1 = v1.toStr();
                        String s2 = v2.toStr();
                        return new VString(s1 + s2);
                }

                throw new InterpreterError("illegal types to + operator");
        }

        @Override
        public ASTType typecheck(Environment<ASTType> e) throws TypeCheckError, InterpreterError {
                ASTType l = lhs.typecheck(e);
                ASTType r = rhs.typecheck(e);
                if (l instanceof ASTTInt && r instanceof ASTTInt)
                        return ASTTInt.tint;
                if (l instanceof ASTTString || r instanceof ASTTString)
                        return ASTTString.tstring;
                throw new TypeCheckError("+: arg types not int nor string");
        }

        @Override
        public boolean isFreeOutsideLambda(String id) {
                return lhs.isFreeOutsideLambda(id) || rhs.isFreeOutsideLambda(id);
        }

}